1. Field of the Invention
The present invention relates to a prismatic sealed secondary battery having a case made of a metal material and a method for manufacturing the same, in particular to a technology for suppressing swelling of the case when the battery is being charged, has gone through several charge-discharge cycles, or has been left in an elevated temperature environment.
2. Related Art
The spread of sealed secondary batteries as electrical power sources for portable devices, such as PDAs, is remarkable in late years. Especially, prismatic sealed secondary batteries (hereinafter referred to simply as “prismatic batteries”) have been brought to attention due to their increased space efficiency and such when attached to devices.
The prismatic batteries have a structure in which the open end of the case is sealed by a sealing cap with an electrode assembly and like housed in the prismatic tubular case having a bottom, and the prismatic tubular case is made of a metal material. For prismatic batteries having such a structure, as with other forms of batteries, there is a demand for compatibility between higher capacity and a reduction in size and weight, and the prismatic batteries are required to have as large an electrode assembly as possible within specified outside dimensions. Therefore, a case with thin plate thickness of 0.2-0.4 mm is generally used for the prismatic batteries. In addition, when the prismatic batteries are attached to devices, there is little clearance between the outer surface of the case and the device having the battery attached thereto in order to improve space efficiency and like of the device.
As to sealed secondary batteries including the prismatic batteries, the internal pressure increases due to, for example, the expansion of gases generated from electrode plates, which are components of the electrode assembly, and/or the swelling of the electrode assembly when the sealed secondary batteries are being charged, have gone through several charge-discharge cycles, or have been left in an elevated temperature environment. When the internal pressure of the batteries exceeds a given value, it is sometimes the case that the swelling occurs in main planes of the case. This is especially pronounced in the prismatic batteries. By taking into consideration the fact that, when the sealed secondary battery is attached to a device, there is little clearance between them, such swelling of the case becomes a problem even if the amount of the swelling is small. Consequently, it is desired that the case swelling be suppressed as much as possible.
In order to suppress such case swelling of the prismatic batteries, various measures are being taken. The followings are examples of such measures that have been developed: a technology for improving the strength of the case by hardening it with use of a laser beam irradiation on the case surface (Japanese Laid-Open Patent Application Publication No. 2002-110108); and a technology for creating grooves arranged in an X pattern on the case by press working before the electrode assembly is housed therein and herewith improving the strength of the case (Japanese Laid-Open Patent Application Publication No. 2001-57179).
A method of suppressing the case swelling by so-called a laser forming process has also been studied. In the process, a laser beam is irradiated on the main planes of the case in order to cause the irradiated parts to melt. Subsequently, the irradiated parts cool off and then resolidify. Thus, by forming parts composed of a recoagulated structure, the case is dented inward centering around the laser irradiation signatures.
However, the technology disclosed in the above-mentioned Japanese Laid-Open Patent Application Publication No. 2002-110108 is, in general, hardly effective for Al—Mn alloys (Japanese Industrial Standard: 3000-series) used for cases of the prismatic batteries. This is because the 3000-series Al alloys have characteristics of little hardening by heat treatments, including the treatment by laser beam irradiation. If a type of Al alloys which can be easily hardened by quenching (2000-, 6000-, and 7000-series) is used for the case, a problem of less weldability will arise since these Al alloys contain Mg in the compositions. For this reason, these Al alloys cannot be practically used for the case.
The technology disclosed in the above-mentioned Japanese Laid-Open Patent Application Publication No. 2001-57179 requires the press working on the case before the electrode assembly is placed therein, and the size of the electrode assembly has to be set smaller by the depth of the grooves. Accordingly, this technology does not satisfy the requirement that the prismatic batteries must have the highest possible capacity within the limited outside dimensions.
Compared to the technologies disclosed in the former two documents (Japanese Laid-Open Patent Application Publications No. 2002-110108 and 2001-57179), the above-mentioned technology using the laser forming process excels in accomplishing the objectives for ensuring a high battery capacity and suppressing the case swelling. However, these objectives can only be achieved when a plurality of linear laser irradiation signatures aligned parallel to each other are created on the main planes of the case. Therefore, this technology remains a problem in terms of working efficiency. That is, this method requires at least a plurality of parallel laser irradiation signatures to be created so that a sufficient area of the main planes of the case is subjected to heat strain. Herewith, this method takes a large number of manufacturing stages required for the process, and therefore the method requires improvement in terms of mass production.
Additionally, in the above technology using the laser forming process, each laser irradiation signature remains conspicuous, which is undesirable for quality in appearance. Furthermore, the depressions formed in this method have large dip angles, and this may cause wrapping film of the case to lift from the case surface.